1. Field of the Invention
The present invention relates to a lens barrel and an imaging device.
2. Description of Related Art
A lens barrel of an imaging device, such as a digital camera and video camera, comprises an optical system guiding an image of an object, an imaging element disposed in an optical axis of the optical system, which may be a CCD, and light control means for adjusting an amount of light guided to the imaging element along the axis of the optical system.
In many cases, an iris, which mechanically adjusts an aperture, is used as the light controlling means. However, use of the iris is disadvantageous for a reduction in size of the lens barrel since blades of the iris and a mechanism for driving the iris blades occupy a considerable space.
Therefore, there has been proposed an imaging device using a liquid-crystal light control element which occupies a relatively small space. For instance, such an imaging device is disclosed in JP-A-2002-82358.
As shown in FIG. 3, a liquid-crystal light control element 16 comprises a liquid crystal layer 1604 where a large number of liquid crystal molecules 1602, each rod-shaped, are sealed in such that the molecules 1602 are capable of being tilted together with longitudinal directions or axes of the respective molecules 1602 parallel to one another. By application of a voltage to the liquid-crystal light control element 16, the direction of the axes of the rod-shaped liquid crystal molecules 1602 that are parallel to one another is varied, changing an angle of the axes of the liquid crystal molecules 1602 with respect to the direction of the thickness of the liquid crystal layer 1604.
The smaller an angle made between the axes of the liquid crystal molecules 1602 and the direction of the light path is, the more light is allowed to pass through the light control element 16 (or, the higher the light transmission is). That is, the closer to a right angle the angle made between the axes and the direction of the light path is, the less light is allowed to pass through the light control element 16 (or, the lower the light transmission is).
An optical system 14 of the imaging device comprises a plurality of lenses 1402, 1404 and a cylindrical body 1406 holding the lenses 1402, 1404. In terms of optical design, it is often the case that when the diameters of the lenses 1402, 1404 and the dimensions of the lenses 1402, 1404 in the direction of an optical axis of the optical system 14 are reduced to decrease the outer diameter and length of the cylindrical body 1406 in order to reduce the size of the optical system 14, rays of light guided from the cylindrical body 1406 to an imaging element 18 respectively diverge away from the optical axis of the lenses 1402, 1404 as they travel toward the imaging element 18.
Therefore, to reduce the diameter and length of the cylindrical body 1406 of the lens barrel 10 and dispose a liquid-crystal light control element 16 as light controlling means between the cylindrical body 1406 and the imaging element 18 is significantly advantageous for a reduction in size of the lens barrel 10, but this invites the drawback that light rays passing through the liquid-crystal light control element 16 are not parallel to one another, but respectively diverge away from the optical axis of the lenses 1402, 1404.
Accordingly, when the light rays diverging away from the optical axis of the lenses pass through the liquid-crystal light control element 16, with the liquid crystal molecules 1602 inclined to make an angle with respect to the direction of the thickness of the liquid crystal layer 1604, as shown in FIG. 14, the angle formed between the transmitted light, that is, light passing through the liquid-crystal light control element 16, and the axes of the liquid crystal molecules 1602 is not uniform over a plane of the liquid crystal layer 1604, but varies in a direction parallel to the plane. Therefore, the amount of passing light also varies depending on the location in the plane of the liquid crystal layer 1604.
When the amount of light passing through the light control element 16 thus varies in a direction along a plane of an imaging area 1802 of the imaging element 18 due to the reduction in size of the optical system 14, there is caused an inconvenience that an unnatural difference or variation in brightness occurs in an image taken by the imaging element 18.